Cocured polymer composite structures are used in advanced fighter aircraft to reduce the part count and improve the structural efficiency. Wing control surfaces are the important primary structure of an aircraft. They are operated by the actuators. Advanced fighter aircraft wing primary control surfaces like Flaperon and Aileron are designed with skin stiffeners construction, which are made with intermediate modulus (IM7/T800) carbon fibre with 8552 epoxy resins which have 15% more stiffness and strength as compared to standard modulus carbon fibre epoxy composites. The construction of Flaperon Test Box is having cocured bottom skin assembly along with titanium hinge and horn which is connected to hydraulic actuator. Boundary members of the Flaperon Test Box are made with aluminium alloy BSL 68 T651 which is fastened with cocured bottom skin assembly. The main challenge lies in positioning of mid spar and ribs on bottom skin along with shims and cure the component. This work focuses on development of cocured bottom skin assembly (skin is cocured along with composite J-Section mid spar and J-Section transverse ribs) using cocuring technology which reduces the fastener joints, and top skin was fastened to bottom skin stiffeners. With these unique design features, portion of wing control surface, Flaperon Test Box was designed and successfully manufactured to validate these structural features before using on the advanced fighter aircraft. The present structure is a part of technology demonstration for advanced fighter aircraft. The Flaperon Test Box is designed to demonstrate cocuring technology. Ultrasonic scan of composite parts showed good consolidation vindicating the processes employed.